Electroplating is a method to produce a metal film or metal foil by electrolyzing a solution which contains metal ions (hereinafter referred to as an electrolytic solution). For example, an electrolytic zinc-coated steel plate used for a vehicle body is such that a steel plate is immersed in an aqueous solution in which zinc ions are dissolved and the zinc ions are reduced by using the steel plate as a cathode to form a zinc film on the steel plate. Further, electroplating includes not only a process in which a metal film is formed on a conductive substrate such as a steel plate but also a process in which, for example, as found in production of electrolytic copper foil, a cylindrical and rotatable cathode is partially immersed in an aqueous solution containing copper ions, a copper thin film is continuously deposited on the surface of the cathode, with the cathode being rotated, and at the same time, the thin film is peeled from one end of the cathode to produce copper foil. As described above, metals to be electroplated include such metals as copper, zinc, tin, nickel, cobalt, lead, chromium, indium, platinum group metals (platinum, iridium, ruthenium, palladium, etc.), precious metals (silver or gold), other transition metal elements, metals collectively called rare metal or critical metal, or their alloys. The above-described anode for electroplating is available in various shapes depending on a metal film and metal foil to be produced, however, in terms of materials thereof, the anode includes a carbon electrode made of graphite or glassy carbon, etc., a lead alloy electrode, a platinum-coated titanium electrode and an oxide-coated titanium electrode. In particular, in electrogalvanizing and production of electrolytic copper foil which use a sulfuric acid based acidic aqueous solution containing metal ions, used is an oxide-coated titanium electrode in which a titanium substrate is coated with a catalytic layer that contains iridium oxide. Further, in electroplating which uses a chloride based aqueous solution that contains metal ions, used is an oxide-coated titanium electrode in which a titanium substrate is coated with a catalytic layer that contains ruthenium oxide. The inventor of the present application has disclosed in Patent Literature 1 and Patent Literature 2 an electrode which has a catalytic layer containing crystalline or amorphous iridium oxide formed on a conductive substrate, as an oxide-coated titanium electrode which is used for the above-described anode for electroplating. In addition, an oxide-coated titanium electrode used in electroplating is disclosed, for example, in Patent Literature 3 and Patent Literature 4. In the Patent Literatures described above, examples of electroplating which mainly uses an acidic aqueous solution such as a sulfuric acid based acidic aqueous solution are described. However, electroplating may be performed by using a substantially neutral aqueous solution or an alkaline aqueous solution. The electroplating which has been described in the present invention covers such electroplating that uses an aqueous solution of a wide range of pH, from acidic to alkaline, and such electroplating that uses a chloride based aqueous solution.
Energy consumed in electroplating is the product of electrolytic voltage and amount of electricity used for electrolysis, and an amount of metal deposited on a cathode is proportional to the amount of electricity. Therefore, electric energy per unit weight necessary for a metal to be electroplated (hereinafter, referred to as electric energy consumption rate) is decreased in accordance with a decrease in electrolytic voltage. The electrolytic voltage is a difference in potential between an anode and a cathode, and a reaction of the cathode is different depending on a metal to be electroplated at the cathode and a potential of the cathode is also different depending on a type of the reaction. On the other hand, a main reaction of the anode is production of chlorine where an aqueous solution containing chloride ions at high concentrations is used as an electrolytic solution. Excluding the above case, a main reaction is oxygen evolution when used in an aqueous solution of a wide range of pH. For example, in production of electrolytic copper foil by electroplating, a sulfuric acid based acidic aqueous solution is used, and in gold electroplating, an alkaline aqueous solution is used. In these electrolytic solutions, a reaction of the anode is oxygen evolution; alternatively, a main reaction of the anode is at least oxygen evolution. A potential of the anode when performing electroplating will vary depending on a material used in the anode. For example, when a material having a low catalytic activity for oxygen evolution and/or chlorine evolution which is a reaction of the anode is compared with a material having a high catalytic activity, the higher the catalytic activity, the lower the potential of the anode. Therefore, where electroplating is performed by using the same type of an electrolytic solution, in order to decrease an electric energy consumption rate, it is critical and necessary to use a material high in catalytic activity for the anode so as to decrease a potential of the anode.
Further, an anode used for electroplating is required not only to have a high catalytic activity for oxygen evolution and/or chlorine evolution but also to have a low catalytic activity for a reaction which may take place on an anode other than these main reactions (hereinafter, referred to as a side reaction), contrary to the case of the main reactions. The previously described sulfuric acid based acidic aqueous solution used, for example, in production of electrolytic copper foil contains lead ions as an impurity in addition to copper ions which are an essential component of the electrolytic solution. There is a case that the lead ions may be oxidized on the anode and deposited on the anode as lead dioxide. The above-described deposition of lead dioxide on the anode will take place at the same time with oxygen evolution which is a main reaction of the anode. Lead dioxide has a low catalytic activity for oxygen evolution and, therefore, inhibits oxygen evolution on the anode and raises a potential of the anode, thereby resulting in an increase in electrolytic voltage. The above-described deposition and accumulation of a metal oxide on the anode by a side reaction increase an electrolytic voltage and also cause decreasing the service life and durability of the anode.
Due to the above-described reasons, the anode for electroplating which uses an aqueous solution as an electrolytic solution is required to have the following features: 1) a high catalytic activity for oxygen evolution and/or chlorine evolution; 2) a low catalytic activity for a side reaction which makes deposition of a metal oxide on the anode and also a side reaction which allows the deposits to adhere and accumulate on the anode even when no metal component is contained; 3) therefore, there is a high selectivity for a main reaction; 4) as a result, the anode is low in potential, in other words, overvoltage for a reaction of the anode is low and no increase in potential of the anode is caused by effects of a side reaction even when electroplating is continued; 5) therefore, the electrolytic voltage is low and the low electrolytic voltage is maintained, by which the electric energy consumption rate for electroplating a target metal is decreased; 6) at the same time, no reduction in service life and durability of the anode is caused by the effects of a side reaction; and 7) a material which is high in durability for a main reaction is used. With regard to the above-described requirements, the inventor of the present application has already disclosed in Patent Literature 2 the anode in which a catalytic layer containing amorphous iridium oxide is formed on a conductive substrate as an anode suitable for electroplating which uses a sulfuric acid based electrolytic solution in production of electrolytic copper foil, etc. Further, in Patent Literature 3, there has also been disclosed the titanium electrode in which a catalytic layer containing amorphous iridium oxide is formed.